Nucleic acids coding for vacuolar invertases, vegetal cells and plants containing said nucleic acids and the use thereof

ABSTRACT

The present invention relates to a nucleic acid molecule encoding a vacuolar invertase and its use for modifying the plant saccharose metabolism, in particular in the vacuole of the cell of a plant storage organ. The present invention also relates to a fragment of the nucleic acid molecule, a vector containing the nucleic acid molecule or a fragment thereof, as well as a host cell, in particular a plant cell that contains the nucleic acid molecule or a fragment thereof. The present invention also comprises a transgenic plant, methods for producing a transgenic plant, as well as methods for modifying the saccharose metabolism of a plant, in particular of a plant part.

FIELD OF THE INVENTION

The present invention describes a nucleic acid molecule that encodes a vacuolar invertase and its use for modifying the plant saccharose metabolism, especially in the vacuole of the cell of a plant storage organ. The present invention furthermore relates to a fragment of the nucleic acid molecule, a vector containing the nucleic acid molecule or a fragment thereof, as well as a host cell, in particular a plant cell that contains the nucleic acid molecule or a fragment thereof. The present invention furthermore comprises a transgenic plant, methods for producing a transgenic plant, as well as methods for modifying the saccharose metabolism of a plant, in particular of a plant part.

BACKGROUND OF THE INVENTION

During the storage of sugar beets (Beta vulgaris), i.e., during the period between harvest and further processing, significant saccharose losses occur, which can be attributed mainly to respiration and the saccharose metabolism. The saccharose loss is associated with a reduction in the quality of the sugar beets, as the content of reducing sugars, such as glucose and fructose, increases (Burba, M., Zeitschrift für die Zuckerindustrie, 26 (1976), 647–658). The saccharose breakdown during the storage of sugar beets comprises as a first metabolic step an enzymatic hydrolysis as a result of a vacuolar invertase. Vacuolar and/or cell wall-bound invertase isoforms are also induced by de novo injuries to beet tissue (Milling et al., J. Exp. Bot., 44 (1993), 1687–1694). This means that injuries also result in a saccharose hydrolysis, particularly in order to provide hexoses necessary for wound respiration and the wound-activated oxidative pentaphosphate (OPP) cycle. A precise analysis as to which invertase isoforms are involved in the wound reaction has not been performed, however. Understanding the regulation and the role of the individual invertase isoforms in the injured sugar beet tissue is critical, however, for being able to manipulate the expression of the individual isoforms, especially considering the substantial saccharose losses during the storage period.

BRIEF SUMMARY OF THE INVENTION

The present invention is therefore based on the technical problem of providing methods and means enabling the production of plants, in particular sugar beets, with a modified saccharose metabolism. The specific modification of the vacuolar saccharose metabolism in the plant cell, in particular in a vacuole of this cell, therefore remains an essential basic prerequisite for providing plants, particularly sugar beets, with improved storage stability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an antisense construct according to the invention with the nucleotide sequence according to the invention, the entire VIwit cDNA sequence being SEQ ID No. 2;

FIG. 2 shows a sense construct according to the invention with the nucleotide sequence according to the invention, the entire VIwit sequence cDNA being SEQ ID No. 2;

FIG. 3 shows the induction of Vlwit mRNA (the entire VIwit cDNA sequence being SEQ ID No. 2) over time in injured storage root tissue of sugar beet tissue compared to the induction of PAL. For these experiments, 10 μg total RNA per track were loaded, and the uniform loading was verified by ethidium bromide staining and hybridization with an 18S RNA sample;

FIG. 4 shows the induction of VIwit protein, the entire VIwit cDNA sequence being SEQ ID No. 2, into injured sugar beet storage root tissue;

FIG. 5 shows the induction of VIwit (the entire VIwit cDNA sequence of which is a SEQ ID No. 2) enzyme activity over time in injured sugar beet storage root tissue. The data are stated as means + standard deviation from three different experiments; and,

FIG. 6 shows an RNA blot with VIwit mRNA, the entire VIwit cDNA sequence being SEQ ID No. 2. In this figure, R: root, S: entire sprout; H: hypocotyls;; TR: tap root; YL: young leaf (50% of final size); OL: fully grown leaves; and F: flower. The plants were harvested 1, 4, 6, 8, 14, 16 and 66 weeks (W) after seeding.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the technical problem was solved by identifying and characterizing a plant gene, which encodes a wound-induced vacuolar invertase (VIwit), and the encoded protein. By specifically modulating the activity, in particular by turning off or inactivating this vacuolar invertase induced by injury, saccharose losses can be reduced during storage or after injury. According to the invention, it was demonstrated for the first time that the activity of a plant vacuolar, wound-induced invertase is inhibited by a corresponding antisense construct. According to the invention, it was shown that the saccharose breakdown induced by injury is caused by a single, defined, and specific vacuolar invertase isoform, VIwit. According to the invention, this isoform was cloned. Different segments of the cDNA sequence provided according to the invention may be used for constructing antisense constructs, sense constructs, sense constructs for co-suppression and for hairpin gene constructs according to Smith et al. (2000 Nature 407, 319–320) for turning off the activity of this wound-induced vacuolar invertase.

According to the invention, a preferably isolated and completely purified nucleic acid molecule is provided, selected from the group consisting of:

a) a nucleic acid molecule with a nucleotide sequence shown in SEQ ID No. 1, 2, 3 or in SEQ ID No. 5 or fragment thereof;

b) a nucleic acid molecule with a sequence encoding a protein with the biological activity of an invertase and with a sequence shown in SEQ ID No. 4 or in SEQ ID No. 6, or a fragment thereof;

c) a nucleic acid molecule complementary to a nucleic acid molecule according to a) to b), or a fragment thereof;

d) a nucleic acid molecule available by substitution, addition, inversion and/or deletion of one or more bases of a nucleic acid molecule according to a) to c);

e) a nucleic acid molecule that hybridizes, for example, due to the degeneration of the genetic code, with a nucleic acid molecule according to a) to d) or a fragment thereof.

A nucleic acid molecule or a nucleic acid may be a DNA sequence, for example, cDNA or a genomic DNA sequence, or an RNA sequence, for example, a mRNA sequence, in particular from the sugar beet, Beta vulgaris. The nucleic acid molecule or the nucleic acid preferably is available in isolated and purified form.

The invention also comprises modified nucleic acid molecules available, for example, by substitution, addition, inversion and/or deletion of one or more bases of a nucleic acid molecule according to the invention, in particular within the encoding sequence of a nucleic acid, i.e., also nucleic acid molecules that that may be called mutants, derivatives, or functional equivalents of a nucleic acid molecule according to the invention. Such manipulations of the sequences are performed, for example, in order to specifically modify the amino acid sequence encoded by a nucleic acid. Nucleic acids that encode modified vacuolar invertases may be used, among other things, for transforming agriculturally used plants in order to produce transgenic plants. Specific sequence modifications also may have the goal of providing suitable restriction site or to remove unnecessary restriction sites. In the process, the nucleic acid molecules according to the invention are inserted into plasmids and are subjected, using standard methods, to the microbiology/molecular biology of a mutagenesis or sequence modification by recombination. For example, methods for in vitro mutagenesis, primer repair methods, as well as restriction and/or ligation methods are suited to bring about insertions, deletions, or substitutions, such as transitions and transversions (cf. Sambrook et al., 1989, Molecular Cloning: A Laboratory Manual, 2nd edition (1989), Cold Spring Harbor Laboratory Press, New York, USA). Sequence modifications also may be achieved by the attachment of natural or synthetic nucleic acid sequences. Examples of synthetic nucleic acid sequences are adaptors or linkers, which are added, among other things, also for linking nucleic acid fragments to these fragments.

The present invention also comprises nucleic acid molecules that hybridize with one of the previously described nucleic acid molecules according to a) to d). The expression “nucleic acid molecule that hybridizes with one of the previously described nucleic acid molecules according to a) to d)” used in connection with the present invention means a nucleic acid molecule or a nucleic acid which hybridizes with a nucleic acid molecule or nucleic acid according to a) to d) under moderately stringent conditions. The hybridization, for example, may take place with a radioactive gene probe in a hybridization solution (25% formamide; 5×SSPE; 0.1% SDS; 5×Denhardt solution; 50 μg/ml herring sperm DNA; with respect to composition of individual components, cf. Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd edition (1989), Cold Spring Harbor Laboratory Press, New York, USA) for 20 hours at 37° C. Unspecifically bound probe is then removed by repeated washing of the filters in 2×SSC/0.1% SDS at 42° C. Preferably the filters are washed with 0.5×SSC/0.1% SDS, especially preferably with 0.1×SSC/0.1% SDS at 42° C. In a preferred embodiment, these nucleic acid molecules have at least 60%, preferably at least 80%, 85%, 90%, 95%, 98%, and especially preferably at least 99% homology (identity) at the nucleic acid level with respect to each other.

The present invention also comprises nucleic acid molecules that encode a polypeptide or protein with the activity of a vacuolar invertase whose sequence has at least 40%, preferably at least 60%, especially preferably at least 70%, 80%, 90%, 95%, especially 99% homology with a polypeptide or protein that is encoded by a nucleic acid with a sequence shown in SEQ ID No. 1,2,3 or in SEQ ID No. 5.

In connection with the invention, the expression “at least 40%, preferably at least 60%, especially preferably at least 70%, 80%, 90%, 95%, especially 99% homology” refers to a sequence match at the amino acid level that can be determined using known methods, for example, computer-based sequence comparisons (Basic local alignment search tool, S. F. Altschul et al., J. Mol. Bio. 215 (1990), 403–410).

The term “homology”, known to one skilled in the art, designates the degree of relationship between two or more polypeptide molecules determined by the match between the sequences, whereby a match may be both an identical match as well as a conservative amino acid exchange. The percentage of “homology” is derived from the percentage of matching regions in two or more sequences with consideration of gaps or other sequence peculiarities.

The expression “conservative amino acid exchange” means the exchange of an amino acid residue for another amino acid residue, whereby this exchange does not result in a change of the polarity or charge at the position of the exchanged amino acid, for example, the exchange of a non-polar amino acid residue for another non-polar amino acid residue. Conservative amino acid exchanges in connection with this invention include for example: G=A=S, I=V=L=M, D=E, N=Q, K=R, Y=F, S=T, G=A=I=V=L=M=Y=F=W=P=S=T.

The homology between polypeptide molecules related to each other can be determined using known methods. As a rule, special computer programs with algorithms taking into account the special requirements are used. Preferred methods for determining homology first generate the greatest match between studied sequences. Computer programs for determining the homology between two sequences include, for example, the GCG program suite, including GAP (Devereux, J., et al. Nucleic Acids Research, 12 (12) (1984), 387; Genetics Computer Group University of Wisconsin, Madison (Wis.)); BLASTP, BLASTN, and FASTA (Altschul, S. et al., J. Molec Bio 215 (1990), 403–410). The BLASTX program is available from the National Centre for Biotechnology Information (NCBI) and from other sources (Altschul S., et al., BLAST Manual, NCB NLM NIH Bethesda, MD 20894; Altschul, S. et al., J. Mol. 215 (1990), 403–410). The known Smith Waterman algorithm also can be used to determine homology.

Preferred standard parameters for the amino acid sequence comparison include, for example: Algorithm: Needleman and Wunsch, J. Mol. Biol 48 (1970), 443–453; Comparison matrix: BLOSUM 62 by Henikoff and Henikoff, Proc. Natl. Acad. Sci. USA, 89 (1992),10915–10919; Gap penalty: 12; Gap length penalty: 4; Similarity threshold: 0.

The GAP program also is suitable for using the previously described parameters.

In addition, other algorithms, gap opening values, gap extension values, and comparison matrices may be used, including those described in the program handbook of the Wisconsin Suite, version 9 (September 1997). The selection of programs depends both on the comparison to be performed and whether the comparison is performed on sequence pairs, in which case GAP or Best Fit are preferred, or whether it is performed on a sequence and an extensive sequence database, in which case FASTA or BLAST are preferred.

The present invention also relates to a preferably isolated and completely purified protein that can be obtained by expression of a nucleic acid molecule according to the invention or fragment thereof in a host cell. The protein preferably has the same vacuolar invertase properties as the protein that is encoded by a nucleic acid molecule with a sequence shown in SEQ ID No. 1, 2, 3 or SEQ ID No. 5. In order to determine the activity of such a protein, the saccharose cleavage experiments described in Example 6 may be performed.

In connection with the present invention, the expressions “with the biological activity of an invertase” and “activity of a vacuolar invertase” mean that a polypeptide or protein is able to cleave saccharose or glucose under suitable conditions in vitro or in vivo, for example, in the test described in Example 6 for determining the invertase activity.

The present invention also comprises isolated and completely or partially purified monoclonal or polyclonal antibodies or their fragments that react with a protein according to the invention specifically or with such an affinity that, for example, a detection of this protein is possibly.

The invention furthermore relates to a construct containing a nucleic acid according to the invention or a fragment thereof under control of expression regulation elements. In connection with the present invention, a “construct”, which may also be called a vector herein, means the combination of a nucleic acid according to the invention or a fragment thereof with at least one additional nucleic acid element, for example, a regulator element. Examples of such regulator elements are constitutive or inducible promoters, such as the E.coli promoter araBAD (Carra and Schlief, EMBO J., 12 (1993), 35–44) for expression in bacteria, the yeast promoter PMA1 (Rentsch et al., FEBS Lett., 370 (1995), 264–268) for expression in fungal systems, and the viral CaMV35S promoter (Pietrzak et al., Nucl. Acids Res., 14 (1986), 5857–5868) for expression in plants.

In particular, storage-organ-specific promoters of the beet are used according to the invention, so that the nucleic acid according to the invention is expressed in a desired target organ, in particular in the beet organ. Naturally, time-, development, or other tissue-specific regulator elements may be used also. An especially preferred embodiment comprises the use of wound-activated promoters of the beet, so that the nucleic acid according to the invention is only expressed in the case of an injury.

In addition, the nucleic acid or fragment may be provided with a transcription termination signal. Such elements have been described already (cf., for example, Gielen et al., EMBO J., 8 (1984), 23–29). The transcription start and termination regions may be both native (homologue) or foreign (heterologue) with respect to the host organism. The sequence of the transcription start and termination regions may be of synthetic or natural origin or may contain a mixture of synthetic and natural components. In an especially preferred embodiment of the invention, the construct is a plasmid.

The nucleic acid or fragment may be present in the construct, in particular in a plasmid, both in antisense as well as in sense orientation to the regulator element(s). If the nucleic acid according to the invention or its fragment are present, for example, in sense orientation to the at least one regulator element, for example, a promoter, it may inhibit or reduce the endogenous vacuolar invertase activity by way of co-suppression effects, especially after transformation and integration, also with higher number of copies, into the genome. If the nucleic acid or fragment is present in the construct in antisense orientation to the at least one regulator element, the construct may be inserted, for example, into the genome of a plant host cell and may lead to a suppression of the formation of the plant's own vacuolar invertase. The term “genome” here means the genetic makeup located both in the mitochondria, plastids, or nucleus of a cell.

A preferred embodiment of the present invention therefore comprises a construct that includes a nucleic acid according to the invention or a fragment thereof in antisense orientation to a promoter, whereby the expression of a vacuolar invertase is inhibited in a host cell containing the construct. The nucleic acid fragment hereby preferably includes fragments with a minimum length, which enable the hybridization to endogenous invertase mRNA or DNA, for example, at least 10 nucleotides, preferably at least 50 nucleotides, especially preferably at least 200 nucleotides. The construct that contains a nucleic acid according to the invention or a fragment thereof can be inserted into a host cell and can be transcribed there into a non-translatable RNA (antisense RNA) that is able, by binding to an endogenous gene for a vacuolar invertase or to the mRNA transcribed from it, to inhibit the expression of this endogenous gene.

Another preferred embodiment of the invention relates to a construct that contains a nucleic acid according to the invention or a nucleic acid fragment according to the invention in sense orientation to a regulator element, for example, a promoter, which is followed by another, identical or different nucleic acid according to the invention or another identical or different nucleic acid fragment according to the invention in antisense orientation to it. This arrangement enables the formation of a double-strand RNA able to induce the breakdown of the endogenous RNA.

In another preferred embodiment of the invention, a construct is provided that contains a nucleic acid according to the invention in sense orientation to a regulator element, for example, a promoter. The insertion of such a construct is able to inhibit or reduce the transcription of an endogenously present gene for the vacuolar invertase via the co-suppression effect. This method has been described, for example, by Napoli et al. (The Plant Cell (1990), 2, 279 to 289) or in U.S. Pat. Nos. 5,034,323, 5,231,020, and 5,283,184. Such a sequence arranged in sense orientation also may be used in one embodiment of the invention as a fragment with the minimum lengths mentioned for the previously mentioned antisense fragments.

In another embodiment according to the invention provides that the endogenously present gene or its two alleles of the vacuolar invertase are deactivated or modified in their activity by a mutation of this gene or of the alleles. This may be accomplished, for example, by transposon-induced mutagenesis or homologue recombination using exogenously inserted nucleic acids according to the invention.

Finally, it is also possible according to the invention to inhibit the expression of endogenous vacuolar invertase genes with catalytic RNA molecules or ribozymes; and according to this embodiment, it may be provided that ribozyme sequences are integrated within antisense RNA sequences according to the invention of this invention in order to obtain an RNA-cleaving activity. The production and use of such target-specific ribozymes are described, for example, by Haseloff et al. (Nature (1988), 334, 585 to 591).

The previously mentioned constructs for the modification, in particular repression, inhibition, or suppression, of endogenously present activity of the invertase or invertase gene must not necessarily contain completely identical, full length nucleic acids according to the invention. Rather, it may be sufficient in many embodiments that the nucleic acids used according to the invention, be it during the production of antisense constructs or co-suppression constructs, contain only fragments, i.e., partial sequences, of the complete sequence according to the invention for the vacuolar invertase. The same is true for the degree of identity, which may well be lower, in particular significantly lower than a 100% identity.

Suppression of the expression of the wound-induced vacuolar invertase isoform is achieved according to the invention also with antisense constructs, sense constructs, or hairpin constructs (Smith et al. (2000 Nature 407, 319–320)). In a preferred embodiment, these constructs are cloned using the entire VIwit cDNA sequence (SEQ ID No. 2) or selected regions thereof (SEQ ID No. 1 or 3). In one embodiment, each construct can be brought to expression under the control of the CaMV 35S promoter; in another embodiment under the control of a wound-induced promoter, or in yet another embodiment under the control of a beet-specific promoter. Sequence sections of, for example, at least 10 nucleotides, preferably at least 50 nucleotides, especially preferably at least 200 nucleotides, are used for the antisense and sense constructs.

For the production according to the invention of hairpin constructs according to Smith et al. (2000 Nature 407, 319–320), preferably sequence sections of, for example, at least 10 nucleotides, preferably at least 50 nucleotides, especially preferably at least 200 nucleotides of the VIwit isoform according to the invention are ligated on the 3′ side with either a spacer DNA or, in a preferred embodiment, with an intron. Then the sequence that is complementary to the first sequence section is ligated to this on the 3′ side in antisense orientation. The intron or spacer sequences used in the aforementioned hairpin construction are here called additional hairpin sequences.

In another preferred embodiment of the invention, the plasmid contains a replication signal for E. coli and/or yeast, and a marker gene that permits a positive selection of the host cells transformed with the plasmid. If the plasmid is inserted into a plant host cell, additional sequences, which are known to one skilled in the art, may be required, depending on the insertion method. If the plasmid, for example, is a derivative of the Ti or Ri plasmid, the nucleic acid to be inserted or the fragment thereof must be flanked by T-DNA sequences that enable the integration of the nucleic acid or of the fragment thereof into the plant genome. The use of T-DNA for the transformation of plant cells has been extensively studied and has been described, for example, in EP 120 516; in Hoekema, The Binary Plant Vector System, chapter V (1985), Offset-drukkerij Kanters B. V. Ablasserdam; in Fraley et al., Crit. Rev. Plant. Sci., 4 (1985), 1–46, and in An et al., EMBO J., 4 (1985), 277–287. Once the inserted nucleic acid or fragment thereof is integrated into the genome, it is usually stable there and is also preserved in the progeny of the originally transformed cell.

The sequence integrated in the genome also may include a selection marker that provides the transformed plant cells with resistance to a biocide or an antibiotic, such as kanamycin, G418, bleomycin, hygromycin, or phosphinotricin. The used marker permits the selection of transformed cells versus cells without the transformed DNA.

According to the invention, the vector may be, for example, a plasmid, cosmid, bacteriophage, virus, or liposome.

The invention thus also provides a host cell containing a nucleic acid according to the invention, in particular a nucleic acid with a sequence shown in SEQ ID No. 1, 2, 3 or a nucleic acid with a sequence shown in SEQ ID No. 5, or a fragment thereof, or a construct containing a nucleic acid according to the invention or a fragment thereof. The host cell according to the invention may be a bacterium or a yeast, insect, mammal, or plant cell.

The present invention furthermore relates to a transgenic plant containing in at least one of its cells a nucleic acid according to the invention that encodes a protein with the activity of an, in particular vacuolar, invertase or a fragment thereof, or a construct according to the invention. The transgenic plants may be plants of different species, genera, families, orders, and classes, i.e., both monocotyle as well as dicotyle plants, as well as algae, mosses, ferns, or gymnosperms. Transgenic plants also may include calli, plant cell cultures, as well as parts, organs, tissues, and harvest or propagation materials of these plants. In the present invention, the transgenic plants are in particular tobacco, potato, tomato, sugar beet, sugar cane, soybean, coffee, pea, bean, cotton, rice, or maize plants.

In connection with the present invention, the expression “in at least one of its cells” means that a transgenic plant contains at least one cell, but preferably a plurality of cells, that contain one or more nucleic acids according to the invention or a fragment thereof or a construct according to the invention which have been integrated in a stable manner. The nucleic acid or a fragment thereof or a construct according to the invention may be integrated both in the nucleus of the cell or in the mitochondrial or plastid genome. The nucleic acid, fragment, or construct preferably is integrated in a location of the cell that does not correspond to its natural position, or is integrated with a number of copies and/or in an orientation that does not correspond to the naturally occurring number of copies and/or orientation.

The present invention also relates to a method for producing a transgenic, preferably fertile plant that comprises the following steps:

-   -   Insertion of a nucleic acid according to the invention, in         particular of a nucleic acid with a sequence shown in SEQ ID No.         1, 2, 3 or a nucleic acid with a sequence shown in SEQ ID No. 5,         or a fragment thereof, or a construct containing a nucleic acid         according to the invention or a fragment thereof, into a plant         cell; and,     -   Regeneration of a preferably fertile plant from the transformed         plant cell, whereby at least one cell of this plant is         transgenic, i.e., contains a nucleic acid according to this         invention integrated in a stable manner and preferably expresses         it, i.e., transcribes the integrated nucleic acid to RNA. The         resulting plant preferably has a reduced invertase activity in         at least one of its vacuoles, and, associated with this, an         increased saccharose stability in the vacuole.

A number of methods are available for inserting a nucleic acid into a plant cell. Most of these methods require that the nucleic acid to be inserted is present in a construct, such as a vector. Vectors, for example, plasmids, cosmids, viri, bacteriophages, shuttle vectors, etc., are known. Vectors often comprise functional units for stabilizing the vector in a host cell and for enabling its replication in it. Vectors may also contain regulator elements with which the nucleic acid is functionally bonded and which enable the expression of the nucleic acid.

In addition to the transformation using agrobacteria, for example, Agrobacterium tumefaciens, there are numerous other methods available. These methods include the fusion of protoplasts, microinjection of DNA, electroporation as well as biolistic methods and virus injection methods. In contrast to transformation using agrobacteria, injection and electroporation do not per se have any special requirements for the vector. Simple plasmids, such as, for example, pUC derivatives, can be used. However, if whole plants are to be regenerated from such transformed cells, the presence of a selectable marker gene is advantageous.

Then whole plants can be regenerated from the transformed plant cells in a suitable medium potentially containing antibiotics or biocides for selection. The resulting plants then can be tested for the presence of the inserted DNA. The transformed cells grow within the plants in the usual manner (cf. McCormick et al., 1986, Plant Cell Reports, 5, 81–84). These plants can be grown as usual and can be crossed with plants having the same transformed genetic traits or other genetic traits. The resulting hybrid individuals demonstrate the corresponding phenotypic properties.

The present invention furthermore relates to a method for the modification of the saccharose metabolism in vacuoles of a plant, in particular the modification in vacuoles of a cell of a storage organ of the sugar beet, whereby a nucleic acid according to the invention, in particular a nucleic acid with a sequence shown in SEQ ID No. 1, 2, 3 or a nucleic acid with a sequence shown in SEQ ID No. 5, or a fragment thereof, are inserted into a plant cell and/or a plant, and then a whole plant is regenerated, whereby in at least one of its cells an expression, i.e., in particular transcription and translation, of the transformed nucleic acid can take place so that a plant with a saccharose breakdown that is reduced in comparison to non-transformed plants in at least one of its vacuoles is obtained.

In order to bring about a modification according to the invention of the properties of the vacuolar saccharose metabolism of a plant, both an antisense formulation, a ribozyme formulation, a double-strand RNA formulation, but also a knock-out formulation or co-suppression formulation may be used. For example, the invention relates to modifications of the saccharose metabolism in the vacuole of a plant, within the context of which the endogenous activity of an invertase is inhibited by antisense expression. In addition, a co-suppression, for example, by inserting several gene copies or constructs containing the nucleic acid sequence isolated according to the invention under the control of a strong constitutively or tissue- and/or time-specifically expressing promoter. Such effects inhibiting the endogenously present invertase activity can be achieved according to the invention by transformation of plant cells with antisense constructs and/or by integration of several sense constructs in order to achieve a co-suppression effect. Naturally, the activity of endogenously present vacuolar invertase genes also can be inhibited via the nucleic acids according to the invention by, for example, transposon-induced insertion into the endogenously present nucleic acid and/or by homologue recombination via homologous recombination.

The present invention furthermore relates to the use of the nucleic acids according to the invention for modifying the saccharose metabolism of plants, in particular, in vacuoles of their storage organs.

The nucleic acids according to the invention, in particular a nucleic acid with a sequence shown in SEQ ID No. 1, 2, 3 or a nucleic acid with a sequence shown in SEQ ID No. 5 also may be used for isolating homologous sequences from bacteria, fungi, plants, animals and/or humans. In order to be able to search for homologous sequences, first gene banks must be created, for example, genomic banks or cDNA banks. With the help of a probe that contains parts of the previously mentioned nucleic acids, sequences then can be isolated from the gene banks. After identification and/or isolation of the corresponding genes, DNA and amino acid sequences can be determined, and the properties of the proteins encoded by these sequences can be analyzed.

The nucleic acids according to the invention also can be used to study the expression of a vacuolar invertase according to the invention in prokaryotic and/or eukaryotic cells. If the previously described nucleic acids, are inserted, for example, into prokaryotic cells, such as bacteria, a RNA sequence, translatable by bacteria, is formed. These bacteria cells therefore can be used for studies of the properties of a vacuolar invertase as well as of its substrates and reaction products. According to the invention, the nucleic acids according to the invention can be used, under control of a regulator element, in antisense direction for inhibiting the expression of an endogenous invertase in prokaryotic and/or eukaryotic cells. Another possible use of these nucleic acids is the production of transgenic crop plants.

The sequence protocol includes:

SEQ ID No. 1 shows a partial cDNA sequence (comprising 686 nucleotides) of the VIwit gene from Beta vulgaris.

SEQ ID No. 2 shows the complete cDNA sequence (comprising 2,337 nucleotides) of the VIwit gene from Beta vulgaris.

SEQ ID No. 3 shows the encoded region (comprising 1,938 nucleotides) of SEQ ID No. 2.

SEQ ID No. 4 shows the complete amino acid sequence (comprising 645 amino acids and derived from SEQ ID No. 3) of the VIwit protein.

SEQ ID No. 5 shows the partial sequence (comprising 7,564 nucleotides) of the genomic VIwit clone of Beta vulgaris.

SEQ ID No. 6 shows a partial amino acid sequence (derived from SEQ ID No. 5 and comprising 284 amino acids) of the VIwit protein according to the invention (corresponds to positions 140 to 423 of SEQ ID No. 4).

SEQ ID No. 7 and 8 show the primers of Beta vulgaris used for amplification and cloning.

SEQ ID No. 9 and 10 show the primers of Beta vulgaris used to produce antisera directed against the N-terminal protein domain.

Other advantageous embodiments can be derived from the secondary claims.

The following examples and associated figures will explain the invention in further detail:

EXAMPLE 1 Plant Material

The diploid Beta vulgaris L. inbred line VV-I/ZR 10676 (KWS Saat AG, Einbeck, Germany) was cultivated under field conditions until bloom (i.e., 66 weeks following sowing). The beets were harvested from the soil in November and stored at 4°C. until April, and then were sown again. For the injury experiments, 24-week-old plants were harvested and stored for 3 months at 6°C. After the plant material had been brought to room temperature, the beet body was decapitated i.e., separated from its stalk. Using a cork drill, tissue cylinders with a diameter of 2 cm were removed from the cortex (4 cylinders per beet body). Immediately after this, the tissue cylinders were cut into slices with a set of fixed razor blades (spaced 2 mm apart). The slices were incubated for different periods of time in a humid atmosphere at room temperature.

EXAMPLE 2 Cloning of cDNA Sequence and Genomic DNA Sequence of the Sugar Beet VI Isoform

After 3 days, RNA was isolated from injured beet tissue. The RNA was amplified with the help of the RT-PCR method using the VIwit-sense primer (SEQ ID No. 7) with the sequence 5′-ATGGTI(G/C)C(G/T)GA(C/T)C(A/G)(A/T)TGGTA(C/T)GA-3′ (I=inosit) and the VIwit-antisense primer (SEQ ID No. 8) with the sequence 5′-TC(A/G)(G/C)T(A/G)TC(A/T)G(A/T)(C/T)TC(A/C/T)CCAA(C/T)CCA-3′, whereby a partial VIwit cDNA (SEQ ID No. 1) was obtained. Reverse MMLV transcriptase was hereby used for the synthesis of the first strand. With the help of the partial cone, a complete cDNA (SEQ ID No. 2) for VIwit (Vacuolar Invertase, wound induced in tap roots) was cloned.

All PCR products were cloned in the Bluescript SK+ vector (Stratagene, Heidelberg, Germany) and sequenced in both directions. The partial cDNA was used as a matrix for synthesizing gene-specific biotinylated probes according to the method by Löw and Rausch (in: A laboratory guide to biotin-labelling in biomolecule analysis (Ed.: Meier, T. and Fahrenholz, F.), 1996, pages 201–213, Birkhäuser Verlag, Basel).

By screening a genomic database that had been generated with the help of the Lambda-ZAP-XhoI-Partial-Fill-In vector kit (Stratagene, Heidelberg, Germany) (Lehr et al., Plant Mol. Biol., 39 (1999), 463–475), genomic clones of the VIwit gene were isolated (SEQ ID No. 5).

EXAMPLE 3 RNA Blot and Southern Blot Analyses

Total RNA was extracted, as described by Logemann et al. (Analyt. Biochem., 163 (1987), 16–20). The total RNA was separated on a 1.4% agarose gel containing 2% formaldehyde, whereby for each track 20 μg of RNA were applied. After capillary transfer to a nylon membrane (Duralon; Stratagene, Heidelberg, Germany) and UV cross-linking, the blots were hybridized with gene-specific probes.

Genomic DNA was isolated from sugar beet leaves according to the method by Murray and Thompson (Nucl. Acids Res., 8 (1980), 4321–4325). After cleavage with restriction enzymes Bg1II, EcoRV, and XbaI, 10 μg of RNA were separated on a 0.7% agarose gel and were then transferred to a nylon membrane. The blots were hybridized using the same probes as previously described.

EXAMPLE 4 Production of Antisera

To detect the VIwit protein, an antiserum against the N-terminal protein domain was produced. For this purpose, the partial VIwit cDNA (SEQ ID No. 1) was amplified with two primers that contained the sequences 5′-GCCGAATCGAGCTCATAAATGGTGTTTGGACAGG-3′ (SEQ ID No. 9) or 5′-GCCGTTAGAAGCTTAGCCGAGGTATCCAACC-3′ (SEQ ID No. 10) (whereby these sequences correspond to the amino acid sequences INGVWTG or GLDTSA). The primers contained a restriction site for the restriction enzyme SacI or HindIII respectively. After cleavage with SacI and HindIII, the PCR products were cloned specifically into the pQE30 vector (Qiagen, Hilden, Germany). After insertion into E.coli strain M15(pREP4), the N-terminal VIwit domain was overexpressed in this strain and then purified using Ni-NTA affinity chromatography. The purified protein was used to produce a polyclonal antibody in rabbits.

EXAMPLE 5 Immunoblot Analysis

The immunoblot analysis was performed according to the method by Weil and Rausch (Planta, 193 (1994), 430–437), whereby, however, 5% dry milk powder was used in place of BSA for blocking, and the immunoblots were developed using the SuperSignal®-West Dura detection kit (Pierce, Rockford, USA). It was found that with control blots the antiserum to VIwit demonstrated no cross-reactivity with other invertase isoforms, for example, cell wall invertases.

EXAMPLE 6 Determination of the Activity of Vacuolar Invertase

To determine the VIwit activity, tissue was extracted in 30 mM MOPS, pH 6.0, 250 mM sorbitol, 10 mM MgCl₂, 10 mM KCl, and 1 mM PMSF. After 10 minutes of centrifugation (6,500×g, 4° C.), the acetone was added to the supernatant (80% final concentration) to precipitate the protein. The protein pellet was suspended in test buffer without saccharose. Aliquots were tested for the activity of the soluble, acidic invertase (VI) in test buffer, whereby the test buffer contained 30 mM saccharose, 20 mM triethanol amine, 7 mM citric acid, and 1 mM PMSF, pH 4.6. The released glucose was enzymatically measured according to the method by Weil and Rausch (Planta 193 (1994), 430–437) in a coupled test with hexokinase and glucose-6-phosphate dehydrogenase.

EXAMPLE 7 Determination of Sugar Concentrations in Tissues

The concentrations of hexoses (glucose and fructose) in the tissue of the beet body were determined according to the method by Stitt et al. (Methods in Enzymology, 174 (1989), 518–552).

EXAMPLE 8 Production of Constructs for Plant Transformation with Wound-Induced Vacuolar Invertase (VIwit)

The VIwit DNA (2337 bp) is present in the pCR®2.1-TOPO® (Invitrogen) vector in “blunt end” cloned form. The VIwit cDNA (or a fragment thereof) is cut from this vector either by digestion with restriction endonucleases or is amplified using a polymerase chain reaction. The cloning in pBinAR is then performed using standard molecular biology methods (restriction digestion of the vector, purification of vector and insert, ligation of vector and insert, as well as transformation of the ligation formulation into a suitable bacteria strain).

FIG. 1 shows the construct for VIwit in antisense orientation under the control of the CaMV 35S promoter in pBinAR (pBin19 derivative).

FIG. 2 shows a construct for VIwit in sense orientation in the same binary vector.

EXAMPLE 9 Transformation of Agrobacterium Tumefaciens

The DNA transfer into the agrobacteria was performed using direct transformation according to the method by Höfgen and Willmitzer (Nucl. Acids Res., 16 (1988), 9877). The plasmid DNA of transformed agrobacteria was isolated according to the method by Birnboim and Doly (Nucl. Acids Res., 7 (1979), 1513–1523) and was analyzed electrophoretically following suitable restriction cleavage.

EXAMPLE 10 Transformation of Sugar Beets to Obtain Transgenic Plants

About one month after the germination of sugar beet seeds in a green house, calli were induced according to the method described by Saunders et al. (J. Plant. Physiol., 124 (1986), 473–479). In the process, three to five centimeter long, young leaves were removed from each plant, disinfected, flushed three times with sterile water, and dried on sterile filter paper. Each leaf was then cut into pieces of approximately 0.25 cm², and the explants obtained in this way were incubated in Petri dishes for 30 days at 30° in the dark and were then transferred into culturing chambers. Four to ten weeks after culture start, white, brittle calli appeared on the leaf explants. Four to six weeks later, the calli were removed and cultivated in 250 ml Erlenmeyer flasks in liquid medium. After approximately two to three weeks, a cell suspension was obtained.

After approximately 3 weeks of cultivation, the cell suspensions were transformed. 10 ml of fresh medium was added to 10 ml of suspension medium. The suspension diluted in this way was distributed into four Petri dishes. From stock cultures of Agrobacterium tumefaciens strains that had been transformed with the created binary vectors, 50 μl were removed and cultivated in 2 ml LB medium that contained rifamicin and tetracycline. The cultures were stirred for two days at 200 rpm and 30° C. The strains were transferred to fresh medium and cultivated overnight under the previously described conditions. The plant cells were infected by adding 50 μl of an Agrobacterium tumefaciens strain from one of the Petri dishes containing the corresponding beet cells. The beet cells and bacteria were cultivated for three days in a culturing chamber in darkness. Then the bacteria were removed from the plan cells. The beet cells washed in this manner were cultivated in Petri dishes on a sheet of sterile Whatman paper and were then incubated for 15 days in the culturing chamber. After three to eight weeks, white calli appeared. After a certain time, sprouts and/or embryons developed from certain calli. As soon as the sprouts had started to develop leaves, these were rooted on medium that contained 1 mg/l of naphthalene acetic acid. After two to six weeks, roots appeared. Once roots had developed, the plants were acclimatized in the greenhouse in humus soil. After another three months, complete plants had developed.

EXAMPLE 11 Wound-induced Formation of VIwit in the Storage Root Tissue

Storage root tissue of field-grown sugar beets was harvested 24 weeks after planting. To produce injured tissue, tissue cylinders with a diameter of 2 cm were cut with a set of razor blades from the cortex. The cuts were incubated in humid air at room temperature for various period ranging from one hour to 15 days. After three days, the tissue showed a slight browning, but no signs of bacterial or fungal infection. The tissue samples were obtained after different incubation times and studied for transcript quantities, the formation of VIwit, and the activity of VIwit. The RNA blot (FIG. 3) with gene-specific cDNA samples shows wound induction of VIwit. VIwit mRNA is induced approximately 24 hours after injury and reaches its maximum after approximately five days. The mRNA remains present over the entire period of 15 days. No signs of a non-specific mRNA breakdown could be determined, which shows that the cellular integrity was not negatively influenced. The transcript quantities of the phenylalanine ammonia lyase (PAL), a gene known for rapid induction after injury, were determined for comparison purposes. The PAL mRNA quantity increased three hours after injury and reached a maximum seven hours later. After five days, the PAL mRNA quantity decreased, whereby a level that is higher than that of uninjured tissue could be maintained, however.

FIG. 4 shows the use of antisera against the central domain of VIwit. The immunoblots reflect the protein quantity of VIwit. The kinetics of the appearing of VIwit proteins correspond to the kinetics of the mRNA (see FIG. 3). Five days after injury, the immunoblot analysis with the VIwit antiserum shows an additional polypeptide with a molecular weight of approximately 33 kDa, which with all probability represents a fragment of VIwit.

The acidic invertase activities in soluble fractions of injured storage root tissue shown in FIG. 5 demonstrate good correlation with the data of FIG. 4. Within the first 24 hours, the vacuolar invertase activity increased by about 50%, whereby a seven-fold increase could be observed 4 days later. It could be demonstrated that VIwit is responsible for maintaining the increased hexose amounts during the wound reaction.

EXAMPLE 12 Expression of VIwit During Plant Development

FIG. 6 shows the transcript quantities of VIwit in various plant organs during development (R root, S entire sprout, H hypocotyl, TR tap root, YL young leaf (50% of final size), OL fully grown leave, F flower). The plants were harvested 1, 4, 6, 8, 14, 20, 26, and 66 weeks (w) after seeding. 10 μg of total RNA were loaded per track and verified through identical loading with ethidium bromide staining and hybridization with an 18S RNA sample.

It could be shown that VIwit is expressed not only in the roots of seedlings, but especially also in leaves during the entire development. 

1. A purified nucleic acid molecule selected from the group consisting of: a) a nucleic acid molecule with the nucleotide sequence of SEQ ID No. 2; b) a nucleic acid molecule with the nucleotide sequence of SEQ ID No. 3; c) a nucleic acid molecule encoding the amino acid sequence of SEQ ID No. 4; and d) a nucleic acid molecule complementary to the nucleic acid molecule according to a), b), or c) wherein the nucleic acid molecule of a), b), or c) encodes a polypeptide having the biological activity of vacuolar wound-induced invertase.
 2. The nucleic acid molecule according to claim 1, wherein the nucleic acid molecule is selected from the group consisting of a), b) and c), and is able to inhibit the expression of a plant vacuolar invertase in a plant host cell when expressed in said cell in antisense orientation.
 3. A vector containing the nucleic acid molecule according to claim 1 operatively linked to at least one expression regulator element.
 4. The vector according to claim 3, characterized in that the nucleic acid molecule is located in antisense orientation to at least one expression regulator element.
 5. The vector according to claim 3, characterized in that the vector is present in the form of a plasmid, cosmid, phage, liposome, or virus.
 6. A host cell, transformed with the nucleic acid molecule according to claim 1 or the vector according to claim
 3. 7. The host cell according to claim 6, wherein said cell is a plant cell.
 8. A transgenic plant, containing in its cells the nucleic acid molecule according to claim 1, or the vector according to claim 3, wherein expression of vacuolar invertase is suppressed and saccharose breakdown is reduced in said plant.
 9. The transgenic plant according to claim 8, characterized in that one or more copies of the nucleic acid molecule, or the vector is/are integrated in a location/locations of the genome different from any native vacuolar invertase gene.
 10. Propagation and harvesting material of a plant according to claim 8, wherein said material contains said nucleic acid molecule.
 11. A method for reducing saccharose breakdown in a plant vacuole comprising inserting the nucleic acid molecule according to claim 1 a), b), c) or a fragment thereof into a plant cell; regenerating a plant from the plant cell; and selecting a transformed plant, wherein transcritpion of said nucleic acid molecule or fragment thereof results in co-suppression of the extension of vacuolar invertase in said plant, and saccharose breakdown in vacuoles is reduced compared to non-transgenic plants of the same species.
 12. A method for reducing saccharose breakdown in a plant vacuole, comprising inserting the nucleic acid molecule according to claim 2 or a fragment thereof into a plant cell; regenerating a plant from the plant cell; and selecting a transgenic plant in which transcription of said nucleic acid molecule or fragment thereof inhibits expression of vacuolar invertase and causes a reduction of saccharose breakdown in plant vacuoles compared to non-transgenic plants of the same species.
 13. The vector of claim 3, further comprising the complementary sequence of said nucleic acid molecule and an additional hairpin sequence, wherein the nucleic acid molecule, the complementary sequence and the additional hairpin sequence form a hairpin construct.
 14. A method for reducing saccharose breakdown in a plant vacuole, comprising inserting the vector according to claim 3 or 13 into a plant cell; regenerating a transformed plant from the plant cell; whereby expression of vacuolar invertase is suppressed in the transformed plant and reduced saccharose breakdown is obtained in vacuoles within cells of the transformed plant.
 15. The method according to claim 12, wherein the plant is a sugar beet.
 16. The vector of claim 3, wherein the expression regulator element is tissue-specific or wound-induced expression regulator element.
 17. The nucleic acid molecule according to claim 1, characterized in that it is a sugar beet DNA or RNA.
 18. The method of claim 14, wherein the plant is a sugar beet.
 19. An isolated nucleic acid molecule comprising the nucleotide sequence of SEQ ID NO:1 or the complement thereof. 